The genetic regulation of tumor metastasis was investigated. A novel gene, nm23, was identified on the basis of its consistent reduced expression in highly metastatic rodent and human tumor cell lines, as well as human breast carcinomas from patients with lymph node metastases. A 17 kDa human nm23-H1 protein was identified, which is highly conserved in evolution and may regulate normal development. The nm23 gene is of potential prognostic and therapeutic importance to the cancer field. Nm23 has been demonstrated to have two characteristics of a suppressor gene for cancer progression, allelic deletion in tumors and suppression of the metastatic phenotype upon transfection. Human lung, colon, renal and breast tumor tissues exhibited allelic deletion of nm23-H1 sequences, as compared to the patient's normal tissue. Nm23-H1 mapped to human chromosome 17 near the centromere, identifying a new site for allelic deletion in cancer. Transfection of murine nm23-1 cDNA into high metastatic potential murine K-1735 TK melanoma cells resulted in the identification of two stable, high expressing lines. Each of these nm23-1 expressing lines produced at least 90% fewer metastases upon injection into mice than randomly chosen control transfectants. A reduced incidence of primary tumor formation was also observed in the nm23-1 transfected lines, as well as alterations in their responsiveness to the cytokine transforming growth factor-beta in vitro. A second human nm23 gene has been identified and sequenced. Expression of the nm23-H2 gene is also reduced in high metastatic potential tumor cells, but appears to be under separate regulatory controls than nm23-H1.